Influence of the memory effect during CO2/CH4 mixed gas hydrate reformation process

Abstract

The study of the memory effect can help to clarify the mechanisms of synthesis and decomposition of the CO2/CH4 mixed gas hydrate for mixed gas separation, CO2 sequestration, and CH4 recovery from unconventional natural gas reservoirs. To examine the impact of the memory effect on the CO2/CH4 mixed gas hydrate synthesis in terms of induction time, gas consumption rate, change of gas components, and separation factor, we conducted initial and second CO2/CH4 mixed gas hydrate synthesis experiments in a porous medium and deionized water system. The influence characteristics of the memory effect on CO2/CH4 hydrate synthesis under the predesigned decomposition temperatures (5 ∼ 25 °C) and pressures (0.5, 1.0, and 1.5 MPa) were investigated under decomposition times of 0.5 and 1.0 h. Compared to the induction time of the initial synthesis, the induction time of the second synthesis was greatly shortened, reaching a maximum of 87.42%. The induction time decreased with decreasing decomposition temperature and pressure in the same decomposition period. The decomposition time had a large effect on the peak value of the induction time during the second synthesis process. Besides, the memory effect was greatly affected by the decomposition time in depressurization cases, and the 0.5 h decomposition time was significantly better than that of 1.0 h. The memory effect had the strongest promotion effect on the second synthesis when the decomposition pressure and time were 1.0 MPa and 0.5 h, respectively. Furthermore, the memory effect improved the nucleation rate of the CO2/CH4 mixed gas hydrate and increased the amount of CO2 hydrate synthesis to 1.3 times that of the initial synthesis. Also, the memory effect influenced the hydrate to exhibit stronger CO2 selectivity, which provides a novel approach for the separation of mixed gases via the hydrate method.